In general, a cord-like heating wire for use in a planar warming apparatus such as an electric blanket or an electric carpet has been hitherto well known. In particular, a cord-like heating wire used frequently in recent years has a configuration called a single-wire type cord-like heating wire in which a heating element wire and a detection element wire are integrated with each other, and the structure thereof is shown in FIG. 2.
The single-wire type cord-like heating wire 1H shown in FIG. 2 includes a winding core 1 composed of a fiber bundle of a polyester fiber or the like, a heating element wire 2 composed of a conductor which is made of copper or a copper alloy and twisted on the outer periphery of the winding core 1 in a spiral manner, a polymer layer 3 formed by extruding a polymer resin onto the outer periphery of the heating element wire 2, a temperature detection element wire 4 composed of a conductor which is made of nickel or the like and twisted on the outer periphery of the polymer layer 3 in a spiral manner, and an insulation coating layer 5 formed by extruding a polyvinyl chloride resin or the like onto the outermost periphery. According to needs, a polyester tape may be twisted in a spiral manner between the temperature detection element wire 4 and the insulation coating layer 5 to provide a barrier layer against shift of a plasticizer from the insulation coating layer 5. In addition, in some single-wire type cord-like heating wire, the heating element wire 2 and the temperature detection element wire 4 are reversely arranged.
In the cord-like heating wire 1H having such a structure, a temperature change due to heating changes the resistance value of the temperature detection element wire 4 made of nickel having a positive temperature coefficient, and the change is converted into an electric signal which is extracted and used for temperature control. Unlike a thermosensitive polymer layer caused to have temperature characteristics by using an ionic conductive agent or the like, the temperature detection element wire 4 composed of a nickel wire has a resistance value and a temperature coefficient which are low but have high accuracy and are stable, so that stable temperature control with high accuracy is achieved over a long period of time.
In the cord-like heating wire 1H, the polymer layer 3 has a unique melting point, and when the cord-like heating wire 1H enters an overheating state, the polymer layer 3 melts and serves as a so-called inter-wire short circuit protection functional material with which the heating element wire 2 and the temperature detection element wire 4 are in contact. This means that, in the single-wire type cord-like heating wire 1H, a control circuit is configured such that the heating element wire 2 and the temperature detection element wire 4 serve as a pair of electrodes which detect a short circuit. In addition, as the polymer layer 3, there is a thermosensitive polymer layer caused to a so-called negative temperature coefficient thermistor (hereinafter abbreviated as “thermistor”) characteristic in which an impedance decreases with temperature rise, a temperature signal different from that of the temperature detection element wire 4 is obtained therefrom, and a control device having a function of preventing local overheating is also realized.
Operations of temperature control and inter-wire short circuit protection of the single-wire type cord-like heating wire 1H are achieved by a temperature control circuit shown in a related art example of FIG. 6. In the temperature control operation, a change in resistance of the temperature detection element wire 4 is divided by resistors R1 and R2, is inputted as a DC input voltage Vi to the minus terminal of a voltage comparator U1 via a smoothing circuit composed of R3 and C2, and is compared with a reference voltage Vref1 corresponding to a preset temperature. A result thereof is outputted from the output terminal of the voltage comparator U1 to drive a power control switch SW to open or close, whereby energization of the heating element wire 2 is controlled. Here, a rectifier diode D4, a voltage reduction resistor R4, an electrolytic capacitor C3, and a three-terminal regulator U2 are used for supplying a low-voltage DC stabilization power supply Vcc=5V to a temperature control section, and GND is a ground for the DC stabilization power supply. In addition, an H point and an N point of an AC power supply are names indicating positions on a circuit diagram and do not include electrical meanings.
For the inter-wire short circuit protection operation, the anodes of diodes D2 and D3 are connected to both ends of the temperature detection element wire 4, respectively, the cathodes of the diodes D2 and D3 are combined and connected to one end of a temperature fuse integral type resistor RF1, and the other end of the temperature fuse integral type resistor RF1 is connected to one end of AC 100 V. The role of D5 in a temperature control circuit diagram of FIG. 6 is to prevent a reverse current from flowing through the inter-wire short circuit protection circuit via the ground GND for the DC stabilization power supply of the temperature control circuit in the case where the N point side of the power supply has a positive cycle.
Here, when the temperature control section is broken to be uncontrollable, the power control switch SW is kept ON to continue energization of the heating element wire 2, whereby the entirety enters an overheating state. Thus, the polymer layer 3 melts at its unique melting point, a short circuit occurs between the heating element wire 2 and the temperature detection element wire 4, a current flows through a path of “AC power supply N point→heating element wire 2→polymer layer 3→temperature detection element wire 4→D2 or D3→RF1→F1→AC power supply H point”, the temperature fuse integral type resistor RF1 is heated, and the temperature fuse thereof is blown within a predetermined time period to disconnect the power supply, whereby a final protection circuit which prevents occurrence of a fire is formed.
When the polymer layer 3 has a thermistor characteristic and a function of detecting an AC impedance with respect to the temperature thereof to prevent local overheating is provided, this is achieved through the following means.
An overheat detection wire is wound on the polymer layer 3 independently of the temperature detection element wire 4. A change in AC impedance between the overheat detection wire and the heating element wire 2 is detected, is inputted to a voltage comparator other than the voltage comparator U1, and is compared with a reference value Vref2 which is set in addition to Vref1. The power control switch SW is driven to open or close based on a result thereof, whereby energization of the heating element wire 2 is controlled.
A temperature signal from the temperature detection element wire 2 is switched between for temperature detection and for overheat detection in a time-division manner by hardware means called a control circuit. The respective signals are inputted to different voltage comparators for temperature control and for overheat prevention, and are compared with reference values for the respective signals. The power control switch SW is driven to open or close based on a result thereof, whereby energization of the heating element wire 2 is controlled.
As described above, the warming temperature control device using the existing single-wire type cord-like heating wire has not only a temperature control function but also a safety protection function, and is configured as a temperature control device whose safety is ensured in terms of configuration.
There are the following related arts for appearance and configuration which are similar as in the above description: JP S48(1973)-066480(A); JP H02(1990)-098088(A); JU H03(1991)-100393(A); JP H05(1993)-003071(A); JP H05(1993)-343169(A); JP H05(1993)-306819(A); JP H06(1994)-005175(A); JP H06(1994)-124771(A); JU H06(1994)-038195(A); JP H07(1995)-216174(A); WO 99/30535; and JP 2015-026458(A).
In recent years, regarding an electric carpet, while the area has been increased, there is a strong market demand for cost decrease by decreasing the wiring density of a cord-like heating wire per unit area. Thus, although an operation with a high watt density of a heating wire becomes common to increase a probability of local overheating, mounting of a local overheating prevention circuit using a negative temperature coefficient thermistor is avoided due to such a local overheating prevention circuit leading to great cost increases or due to limitations by a patent. Thus, products having mounted thereon only a low-cost inter-wire short circuit protection function whose detection capability for local overheating is originally not high flood, so that a temperature control device exposes poor performance, occurrence of an overheating color change or a one-coin-like scorch in a carpet due to local overheating increases, and a risk of a fire is pointed out, which has been a big problem.
A reason why it is not possible to provide an overheating prevention-equipped temperature control device using the above-described thermistor, at low cost is that a process of occurrence of local overheating has been not clear. This point is analyzed in JP 2015-026458(A) which is another application by the present inventors. Here, its outline will be described based on the temperature control circuit diagram of FIG. 6. When the temperature becomes a high temperature exceeding 100° C. as in local overheating, the polymer layer 3 of the single-wire type cord-like heating wire 1H exhibits a decrease in AC impedance close to a thermistor with temperature rise, particularly, even without adding a special additive such as an ionic conductive agent to impart a thermistor characteristic in the case where the material is a polyamide resin, and a leak current flows between the heating element wire 2 and the temperature detection element wire 4 due to overheating, to change the voltage of the minus terminal of the voltage comparator U1, which may adversely affects the temperature control function.
Specifically, in the circuit diagram of FIG. 6, three leak positions, a position between S1 and H1 terminals, a position between the heating element wire 2 at a center portion of the cord-like heating wire 1H and the temperature detection element wire 4, a position between S2 and H2 terminals, are set as parameters, and a relationship between a leak resistance Rx and the input voltage Vi is shown in FIG. 7 obtained by referring to JP 2015-026458(A).
According to FIG. 7,
(1) in the case where a leak position is the S1 and H1 terminals side with respect to the center portion, as the leak resistance Rx due to overheating decreases so that a leak current increases, the voltage Vi inputted to the minus terminal of the voltage comparator U1 increases as compared to the case of no leak, and temperature control works such that the output of the voltage comparator U1 becomes OFF at a temperature lower than a set temperature. Thus, the temperature control has high safety; and
(2) in the case where a leak position closer to the S2-H2 side than to the center portion, as the leak resistance Rx due to overheating decreases so that a leak current increases, the voltage Vi inputted to the minus terminal of the voltage comparator U1 decreases as compared to the case of no leak, and temperature control works such that the output of the voltage comparator U1 becomes OFF at a temperature higher than the set temperature. Thus, the leak current tends to increase, so that a dangerous state leading to overheating is likely to occur.
As described above, in the existing temperature control circuit shown in the temperature control circuit diagram of FIG. 6, in a state where the power control switch SW is ON so that the single-wire type cord-like heating wire 1H is heated, local overheating occurs in a region having a positional characteristic near the S2 and H2 terminals. If a leak current flows through the polymer layer 3 between the heating element wire 2 and the temperature detection element wire 4 of the cord-like heating wire 1H, the leak current decreases the input voltage of the minus terminal of the voltage comparator U1, and acts such that the temperature control output does not become OFF, and positive feedback works so as to further increase the temperature of local heating, which is pointed out to be very dangerous in terms of safety.
For such a problem, in JP H06(1994)-124771(A) and JU H06(1994)-038195(A), a temperature detection element wire and an overheating detection element wire are independently provided. A temperature signal and an overheating signal by a thermistor are separately detected, are inputted to different voltage comparators, and are used for temperature control or overheating prevention. However, there is a drawback that the cord-like heating wire and the temperature control circuit become complicated and cannot be economically provided at low cost.
In addition, in JP H05(1993)-003071(A), the cord-like heating wire has a thermistor function but does not have an overheating detection element wire. A temperature signal included in the temperature detection element wire and an overheating signal by the thermistor are temporally separated and detected through alternate switching of circuit connection by a plurality of transistors. These signals are inputted to different voltage comparators and used for temperature control and overheating prevention. However, in a region where the temperature of the thermistor is low and the impedance is high, there is a drawback that a signal current is low and it is not possible to ensure stable switching operation and detection operation. Also, there is a drawback that the temperature control circuit becomes complicated and cannot be economically provided at low cost.
Further, in WO 99/30535, the cord-like heating wire has a thermistor function but does not have an overheating detection element wire. A temperature signal included in the temperature detection element wire and an overheating signal by the thermistor are temporally separated and detected through division of a path for current in a positive cycle and a negative cycle of an AC power supply by a plurality of diodes. These signals are inputted to different voltage comparators and used for temperature control and overheating prevention, so that both functions are achieved by very simple and economical means. However, there is a drawback that in a region where leak of the thermistor is small, a signal voltage is buried due to insertion loss of a diode, or a signal voltage drifts due to temperature dependency of the diode, so that it is not possible to ensure stable detection operation with high accuracy. In addition, in the above four related arts, there is no description that the leak resistance increases or decreases the input voltage to the voltage comparator depending on the leak occurrence position, and thus it is hard to say that it is effective overheating prevention for all modes of leak occurrence, which is a drawback.